1. Field of the Invention
This invention relates to an improved solvent vapor control system for the minimization of emissions from vapor degreasing and defluxing equipment. More specifically, the invention relates to the use of multiple-stage condensing/heat exchanging within the freeboard region of a vapor degreaser to reduce vapor diffusional losses.
2. Description of the Prior Art Including Information Disclosed Under .sctn..sctn.1.97-1.99
It is generally known and a common commercial practice to employ an organic solvent/cleaning agent in various types of vapor degreasing/defluxing equipment to clean articles of manufacture, deflux electronic circuit boards and the like. It is also generally known and a common commercial practice to employ various volatile organic solvents and, in particular, chlorofluorocarbons, CFCs, as the solvent of choice. However, it is now recognized that the escape of organic solvents and, in particular, the escape of certain CFCs to the atmosphere will potentally contribute to the depletion of the stratospheric ozone layer and contribute to the global warming phenomenon. In view of the above, certain hydroghlorofuorocarbons, HCFCs, and hydrofluorocarbons, HFCs, are now being considered as alternatives to the ozone-depleting CFC solvents. These alternatives are generally more expensive and more physiologically active than commonly used compounds and, in some instances, the proposed alternative compound is also highly volatile with a boiling point at or near room temperature. Consequently, the traditional incentives to reduce vapor losses because of cost and safety considerations are enhanced and of greater criticality when using a low boiling HFC or HCFC as the solvent.
Historically several methods of reducing vapor losses to the atmosphere when using a vapor degreaser have been proposed with varying degrees of success; however, no prior art reference appears to deal specifically with the diffusional losses associated with and caused by the vapor concentration gradient inherently present in the freeboard region of the vapor degreaser. For example, U.S. Pat. No. 2,090,192 uses a single cooling coil to condense vapors within an essentially totally enclosed unit, thus reducing vapor loss to the atmosphere by isolating the vapors from the air. In U.S. Pat. No. 2,816,065 a two-sump, open-top degreaser is disclosed wherein a single refrigerated condenser coil is used at effectively a lower temperature than normal to minimize vapor losses, but again not control of the vapor concentration gradient over the length of the freeboard zone is suggested to reduce diffusional losses.
Also several prior art disclosures have suggested the use of more than one cooling coil or heat exchanger for various reasons, but again, not specifically to reduce the vapor concentration gradient found in the freeboard region of the degreaser. For example, U.S. Pat. No. 2,000,335 suggests the use of two heat exchangers in series within the vapor degreaser. The first heat exchanger is immersed in the hot liquid solvent and is used to heat the water coolant such that the second condensation heat exchanger operates above the dew point preventing water condensation simultaneously with solvent recovery. U.S. Pat. No. 2,650,085 suggests the use of two different temperature cooling coils in a distillation process; however, the process is not a vapor degreaser but rather the distillation and recovery of calcium metal and an alkali metal. In U.S. Pat. No. 3,106,928, the problem of diffusional losses is recognized and the use of a small fan to recycle the vapor/air mixture above the condensing coil to a secondary, external condenser for further vapor condensation is disclosed. In U.S. Pat. Nos. 3,242,057 and 3,242,933 a pair of condenser/heat exchangers each operated at essentially the same temperature are used in a rotating drum and in a conveyer belt automated vapor degreaser system, respectively, wherein the second water-cooled condenser is located at the exit of the automated system.
In U.S. Pat. No. 3,375,177 an open-top vapor degreaser unit that employs a water-cooled primary condenser/heat exchanger to condense the vapors above the boiling sump and an additional refrigerated condenser/heat exchanger above the primary condenser to dehumidify and further reduce vapor loss is disclosed. Again, this reference is void of any suggestion or attempt to control the temperature profile throughout the freeboard zone, such as to reduce the vapor concentration gradient. As such, even this prior art vapor degreaser will exhibit significant vapor losses associated with vapor diffusion.